they're all made up!
well, you asked for it.
the Marvel and DC universe crossover events usually make a point of explaining that they exist in seperate universes though. or at least Marvel VS DC and JLA/Avengers both did.
so too did the Judge Dredd/Batman crossovers.
character cameos like in Soul Calibur don't count. Darth Maul and Wolverine both appear as unlockable characters in Tony Hawk's Pro Skater 3. that doesn't mean they co-exist.
and just because the Montecito casino appears in Heroes, nu Knight Rider and so on, doesn't mean the shhow set there is in that reality. it just means that there's a casino called the Montecito.